Digital high definition video data may be successfully transmitted over terrestrial television channels, as described herein, by partitioning the video data between high and low priority information, and quadrature amplitude modulating the high and low priority data on separate carriers respectively. The modulated carriers are included in a 6 MHZ frequency spectrum and then the combined signal is translated to occupy a standard broadcast channel spectrum. The high priority data is transmitted with relatively high power and the low priority data with relatively low power. High priority data is that video data which is sufficient to reproduce an image, albeit of lesser quality than a high definition image.
The present invention is directed to circuitry for recombining, at the receiver, compressed video data which has been prioritized between relatively high and low priority video data. For purposes of this disclosure the video data will be presumed to be compressed in MPEG like format. What is meant by "MPEG like" is a coding format similar to the standardized coding format being established by the International Organization for Standardization. The standard is described in the document "International Organization for Standardization", ISO-IEC JT(1/SC2/WG1), Coding of Moving Pictures and Associated Audio, MPEG 90/176 Rev.2, Dec. 18, 1990, which document is incorporated herein by reference for description of the general code format.
In the system described herein the video signal is compressed in such fashion to develop blocks of data codewords. The data in respective blocks are divided between high and low priority transmission channels according to the relative importance of the codewords for image reconstruction. The compressed data in the high and low priority channels include, at least in part, variable length encoded data without codeword demarcation.
At the receiver high and low priority data from corresponding blocks is recombined for decompression purposes. The fact that the transmitted data codewords do not have defined boundaries complicates the recombination of high and low priority data.